Viral Capsid Architectures: Helical, Icosahedral, and Complex

Virus particles (virions) package their genome in a protein capsid with three fundamental geometries:

1. Helical – Capsomers spiral around nucleic acid; rod-shaped or filamentous (e.g., tobacco mosaic virus, influenza, rabies). Flexible length accommodates genome size.
2. Icosahedral – 20 triangular faces, 12 vertices; quasi-equivalent capsomer packing (Caspar-Klug theory). Efficient for spherical virions (e.g., adenovirus, herpesvirus, poliovirus).
3. Complex – Neither purely helical nor icosahedral; elaborate structures with tails, baseplates, fibers (e.g., bacteriophages T4, poxviruses).

Enveloped viruses acquire a host-derived lipid bilayer; underlying capsid retains one of these symmetries.